I'm reasonably sure you downgraded the GPU... MW3 also tends to be more memory bandwidth intensive, which has always been a strong suit of AMD/ATI cards compared to NVIDIA.
do you have any idea about the difference in memory bandwidths of these 2 cards???????
Hd 5450- 12.8 gb/sec
9800 GT - 57.6 gb/sec
i upgraded my gpu after a ton of research....so i'm sure of what i'm talking....
Now can u please provide a fix if u can...!!